Definition - What does Brat mean?
Brat is a term for the BDSM role of a bratty submissive or bottom. Unlike the common conception of a submissive, a brat talks back, misbehaves, and is otherwise difficult.
Kinkly explains Brat
Brattiness or brat play is a sub section of Dominant/submissive play. Some people who take the role of brat enjoy being 'put in their place' rather than submitting easily. Likewise, some Dominants enjoy the challenge of getting a brat under control.
Like any form of BDSM play, these roles must be negotiated because these forms of play aren't for everyone and brattiness could be a deal-breaker for some tops.
Some brats use talking back, or phrases such as 'Is that all you've got?' as a way to goad their top into heavier play.